In a conventional pulverized-coal fired boiler in practical use where coal (pulverized coal) is burned with air taken from an atmosphere with a blower, control is made for safe and stable combustion of the coal with the air having an oxygen concentration of about 21% (v/v). Meanwhile, nowadays, global warming suppression calls for urgent attention, which leads to rapidly increasing requests for reducing emissions of carbon dioxide (CO2) discharged together with exhaust gas from the combustion of the coal.
Heat efficiency has been improved in the conventional pulverized-coal fired boiler so as to suppress fuel consumption per energy unit and thus emissions of CO2. However, recent requests for reducing the emissions of CO2 are impossible to meet only by such improvement in efficiency.
Thus, techniques have been developed which remove CO2 from exhaust gas and isolate the removed CO2 for prevention of dispersion thereof to atmosphere. An oxygen combustion boiler has been conceived in this connection.
Specifically, it has been conceived that oxygen (O2) with a concentration of nearly 100% resulting from separation of nitrogen (N2) from air is fed in place of combustion air to a pulverized-coal fired boiler, which drastically reduces N2 in the exhaust gas from combustion of coal to theoretically make the exhaust gas have a CO2 concentration of 90% or more. The CO2 is taken out and reserved, for example, underground or in a sea bottom.
However, in a conventional pulverized-coal fired boiler designed for stable combustion with oxygen having a concentration of the order of 21%, combustion of coal with oxygen having a concentration of nearly 100% will bring about a problem that the boiler cannot stand the damage due to the extremely hot and fierce combustion with the oxygen.
Thus, dilution of the oxygen with some gas is required to attain the oxygen combustion without substantially changing the design of the conventional pulverized-coal fired boiler with established technical stability.
Conceived in this connection is recirculation of exhaust gas. Specifically, the exhaust gas with high levels of CO2 resulting from the oxygen combustion and downstream of the pulverized-coal fired boiler is used as circulating exhaust gas, and the pulverized coal is burned with mixed gas which is a mixture of oxygen with the circulating exhaust gas. As mentioned in the above, the exhaust gas from the oxygen combustion includes substantially no N2 and theoretically has CO2 with a concentration of 90% or more. Thus, the mixing of oxygen with the circulating exhaust gas ensures safe and stable combustion as in the conventional boiler, and makes it possible to withdraw the concentrated CO2 in the exhaust gas through liquefaction or the like for disposal.
Prior art literatures on oxygen combustion boilers as mentioned in the above are, for example, Patent Literatures 1 and 2.